This was a question from an A-Level past paper and the question is pretty straightforward.
I was in a hurry so I couldn't think much on this problem and chose $C$ as my answer, checked it on the answer key, and yes, my answer was correct. So I decided to briefly look at the options once again, and wondered why I chose $C$.
By definition $W=F×d$ , and according to Newton's 1st Law, an object at constant velocity means there isn't any unbalanced force acting on it, thus if $F$ is $0$ , it is said that the object isn't doing any work.
So I changed the scenario a bit, imagine that I'm pushing a car at a constant velocity, according to the definition, I'm not doing work on the car, but that doesn't seem very intuitive to me. I'm clearly exerting my energy to push the car which eventually exhausts me.
Coming back to the first case, I assumed the surface, where the box was sliding, to be frictionless and is placed on a massive vacuum chamber. If my hand was initially pushing that box and after a while, I released it, it'll move at a constant velocity, this suddenly becomes sensible, as there is nothing pushing it and thus no work done.
It seems that when letting go a box with no resistive force, I wasn't doing any work, but when I was pushing the car, I did. It is somehow contradictive and confusing. What concept am I missing? Any concise explanation would be great!